Normal lead styphnate (normal lead trinitroresorcinate) is a well-known primary explosive which has been widely used for the manufacture of percussion primers for small arms ammunition and the like where it first came into use as a replacement for fulminate of mercury. Normal lead styphnate is a very powerful, percussion sensitive explosive which is also quite sensitive to static electricity. The danger of handling it in pure, or relatively pure form, is such that it is normally handled only in small quantities, kept wet, and in rubber containers or "boats." Even with these precautions it has been known to detonate as the result of the stimuli of shock, friction, static electricity, or exposure to sparks or flame.
The normal lead styphnate of this invention is prepared in situ in a wet mixture of styphnic acid (trinitroresorcine) and a lead compound such as lead oxide (litharge), lead hydroxide, basic lead carbonate, or lead carbonate. In the resulting wet mixture the lead styphnate does not show any undesirable sensitivity to initiation by shock, friction, or static electricity and such wet mixtures can be handled in normal primer charging operations with little or no risk of accidental explosion.
As set forth in Davis "The Chemistry of Powder and Explosives," Page 440, Vol. II (1943) John Wiley and Sons Inc., lead styphnate is commonly prepared by adding a solution of magnesium styphnate at 70.degree. C. to a well stirred solution of lead acetate at 70.degree. C. A voluminous precipitate of the basic salt separates. The mixture is stirred for 10 or 15 minutes; then dilute nitric acid is added with stirring to convert the basic salt to the normal salt and the stirring is continued while the temperature drops to about 30.degree. C. The product, which consists of reddish-brown, short, rhombic crystals, is filtered off, washed with water, sieved through silk, and dried. In the processes of filtration and washing undesirable impurities tend to be eliminated.
Other methods of producing normal lead styphnate are described in the patents of Edmund Herz, such as No. 1,443,328 and No. 1,999,728, the latter process having been used commercially for many years in the making of many millions of small arms primers. This latter method is characterized by the preparation of lead styphnate by the interaction of a concentrated solution of magnesium styphnate with a solution of lead nitrate in the presence of free styphnic acid.
The patent to Brun No. 2,239,547 proposes to make a priming mixture by mixing moist mono-basic lead styphnate with moist styphnic acid and other priming mixture ingredients such as a sensitizer, an oxidizer, and an abrasive. However, mono-basic lead styphnate is itself a dangerously sensitive primary explosive and by practicing the Brun invention it would not be possible to achieve the chief advantages of applicant's method as later hereinafter set forth.
When the first attempts were made to produce lead styphnate in accordance with the present invention, commercial styphnic acid was used and the resulting mixtures were found to be so sticky that they clung to the charging equipment and were found to harden to an undesirable degree preventing charging when the wet mixtures were stored for such periods as overnight.
In the commercial manufacture of styphnic acid, resorcinol is first sulphonated with 90% sulphuric acid. After the sulphonation is complete, nitric acid is added, the material nitrated and the styphnic acid precipitated. Some investigators (Franz, Wilkinson and Ehrlich U.S. Pat. No. 2,150,653) feel that when styphnic acid is so manufactured on a commerical scale, small amounts of nitrous and nitric oxides are present during the sulphonation. This, they felt, produces a dark brown material which may include some or all of the nitro derivatives of resorufin, the nitro derivatives of indo-phenol and the nitro derivatives of resazurin.
I have also discovered that styphnic acid produced by one of the commercial processes contains a minor amount of sulphuric acid. When styphnic acid is dissolved in amyl alcohol and the amyl alcohol solution extracted with water several times, the water extracts may be bulked together, evaporated to one-half volume, chilled, and the precipitated crystals of styphnic acid removed by filtration. Successive repetitions of this technique of evaporating the water extract, cooling it to precipitate any remaining styphnic acid, filtering out the crystals, etc. followed by evaporation as far as possible in a steam bath and by evaporation in a vacuum oven at 65.degree. C. resulted in the concentration of a black oily residue. This oily residue was found to have as its principal ingredient sulphuric acid which was undoubtedly carried over from the sulphonation process. Other impurities are also present in the oily residue and are believed to consist of one or more of the impurities referred to in the aforementioned Patent No. 2,150,653 or in a later patent to one of the same inventors, Patent No. 2,246,963.
As will be noted from these patents, it was theorized that the presence of these impurities was helpful in the manufacture of lead styphnate and if the styphnic acid was too pure it was the practice of these inventors to add one or more of such impurities or to modify the process to insure that they would be produced.
Certainly the presence of these impurities had no adverse effects in the formation of lead styphnate by the methods of the prior art for in the precipitation, filtering, washing and drying of the lead styphnate, the impurities or lead salts formed from reactions with such impurities, were readily separated from the dense crystalline normal lead styphnate.
However, in the practice of applicant's process, wherein the normal lead styphanate is precipitated in situ in a priming mixture, there is no opportunity to eliminate the impurities or any products resulting from a reaction with the impurities. Although the impurities do not interfere with the production of the lead styphnate, by remaining in the mixture they profoundly affect the resulting mixture and it may become so sticky that it adheres to tools and charging plates and hence cannot be charged into primers in the usual way and the mixtures may harden up so quickly that they cannot be stored for the required length of time or handled in the normal manner.
Accordingly, when referring to lead styphnate for use in my invention, I refer to a relatively pure styphnic acid in the form of dense crystals of reasonably uniform shape and size and of the light color associated with the pure product. The dark brownish commercial styphnic acid or any other form of styphnic acid containing significant amounts of impurities such as sulphuric acid, the nitro derivatives of resorufin, the nitro derivatives of indophenol and the nitro derivatives of resazurin should not be used.
As has been indicated, there are known prior art methods of producing styphnic acid of the desired purity and the usual commercial styphnic acid may be adequately purified by dissolution and re-crystallization. It has also been noted that with some of the forms of commercial styphnic acid the dry product may be selectively screened to eliminate irregular, large, agglomerated forms. It may be that these irregular, large, agglomerated forms are constituted of a mass of fine particles bound together on a globule of sulfuric acid carried over from the sulphonation process.